Chemical concepts: Functional Groups, Polarity, Solubility

NU FS 372+373

Key concepts

1. Remember common functional groups and be able to recognise them in complex organic molecules (slides 3-12)

2. Understand the concepts of electronegativity and polarity

3. Understand the principles governing solubility and miscibility

4. Be able to define log Pow and understand the singificance of log Pow values

Alkanes: CnH2n+2

Formula	Structure	Name / Uses
CH4		Methane - gas used for cooking.
C2H6		Ethane
C3H8		Propane - heating fuel.
C4H10		Butane - lighter / camping fuel.
C5H12		Pentane
C6H14		Hexane

Hydrocarbon Rings:

Formula	Structure	Name / Uses
C6H12		Cyclohexane - a saturated hydrocarbon with the atoms arranged in a hexagonal ring. In organic chemistry, the presence of Hydrogen atoms is often assumed and this compund can be reprsented by a hexagonal ring:
C6H6		Benzene - an industrial solvent. The Benzine Ring is one of the most important structures in organic chemistry. In reality, its alternate double and single bonds are "spread around" the ring so that the molecule is symetrical. This structure is represented by a hexagon with a circle:
C7H8		Toluene - an important solvent and starter chemical. Using the Benzine Ring, this molecule can also be depicted as:
C10H8		Naphthalene - used in moth balls. This can be depicted as two fused Benzine Rings:

Alcohols: CnH2n+1OH

eneral ormula	Series Name	Details	Examples	Structures
CnH2n+1OH	Alcohols	Alcohols have the OH (hydroxyl) group in the molecule.	CH3OH Methanol wood alcohol C2H5OH Ethanol drinking alcohol C6H5OH Phenol carbolic acid - used as disinfectant

Ethers: (CnH2n+1)2O

			(CH3)2O
			Dimethyl
		Ethers have an O atom	Ether a gas
(CnH2n+1)2O	Ethers	attached to two hydrocarbon chains (or rings).	(C2H5)2O Diethyl Ether a liquid
			used as an
			anaesthetic)

Aldehydes - CnH2n+1CHO

CnH2n+1CHO

Aldehydes have a CHO group attached to a hydrocarbon chain (or ring).

HCHO H Formaldehyde preservative in C labs O CH3CHO Acetaldehyde

H

Ketones - (CnH2n+1)2CO

(CnH2n+1)2CO

Ketones have a CO group attached to two hydrocarbon chains (or rings).

CH3COCH3 Dimethyl Ketone Also known as acetone: nail- varnish remover

Carboxylic Acids

C2H5CO2H

(fatty acids for high n): CnH2n+1CO2H

CnH2n+1CO2H

Fatty Acids contain the CO2H (or COOH) group attached to a hydrocarbon chain or ring.

HCO2H Formic Acid in ant bites and stinging nettles CH3CO2H Acetic Acid vinegar C3H7CO2H Butyric Acid the rancid butter smell

H3C

Esters:

RCO2R’

RCO2R' (R, R' are Hydrocarbon chains or rings).

Esters are similar to Fatty Acids except that the H in the COOH group is another hydrocarbon chain. They are usually very sweet smelling liquids used in perfumes.

CH3CO2CH3 Methyl Methoate essence of pear drops

Amines: RCH2n+1NH2

	Amines have one or more of the Hydrogen atoms in Ammonia (NH3) replaced by a Hydrocarbon chain or ring.
CnH2n+1NH2	Primary Amines have the formula RNH2 Secondary Amines have the formula RR'NH	CH3NH2 Methylamine a pungent, water soluble gas
	Tertiary Amines have the formula RR'R''N.
	(R, R', R'' are Hydrocarbon chains or rings).

Amino Acids H2NRCO2H

CnH2nNH2COOH

Amino Acids have two functional groups: the amine (HN2) group and the fatty acid (COOH) group. Amino Acids combine together to form proteins which are an important component of living organisms.

CH2NH2COOH Glycine the simplest amino acid.

Amides: RCONH2

Primary Amides

propanamide

Benzamide

Secondary Amides - have one hydrogen and one alkyl or aryl group on the N of the amide group.

N-methylethanamide,

Tertiary Amides - have two alkyl or aryl groups attached to the N of the amide group.

N,N-dimethylethanamide,

• POLYAMIDES are secondary amides formed in a condensation reaction between a carboxylic acid and an amine.

  -COOH + H2N-  -CO-NH- + H2O

  
  

• POLYPEPTIDES (proteins) are polyamides formed between amino acids

(a “dipeptide”)

Chlorophyll a (bluish-green and b yellowish-green)

Visible spectrum

Chlorophyll a and b

methyl

aldehyde

pyrrole

ethyl

ketone

ester

alkyl chain

Intramolecular Bonds

• Covalent bonds –

  sharing pairs of

  electrons. Single or H

  NH

  multiple. Number of H

  covalent bonds

  dependent on valency

  
  

• Ionic bonds – between O

H   H C C

H H

strongly electronegative and electropositive atoms eg. salts

R C O-Na+

Na+Cl-

Electronegativity

Electronegativity is the attraction of an atom for the electron cloud that forms the bond between 2 atoms

Least electronegative element

Pauling Scale

Most electronegative element

Polarity

Arises through differences in electronegativity:

★

A ★ B

δ+ ★ δ-

A ★ B

A and B have equal electronegativities..

 nonpolar bond

+

A

B is more electronegative than A..

 polar bond

★-

★B

B is much more electronegative than A..

 ionic bond

Dipole Moment

B is more electronegative than A:

δ+ δ-

d

A B

The product of the magnitude of the charge (δ) either on A or B and the AB bond length (d) is called the dipole moment (µ) of the bond:

µ (AB) = δ x d (SI units are debye (D))

Molecular dipole moments are the vector sum of the dipole moments of all bonds in the molecule

Polar vs. Nonpolar Molecules

H H

H C C H

H H H C C

H

OH

H H H H

Ethane: Non-polar Ethanol: Polar

δδδδ+

δδδ+

δδ+

δ+

An electronegative (electron withdrawing) element X not only polarizes the C-X bond but also draws electrons away from more distant bonds to a diminishing extent. This is the inductive effect, occurring through σ bonds.

CH3——CH2——CH2——CH2——X

δ-

<

<

<

<

Inductive Effects

O2N—<<—CH2—<—Ph

H——CH2——Ph

NO2 is –I i.e. electron withdrawing; this also draws electrons away from the Ph group in the C--Ph bond

Intermolecular Bonds

H

O δ-

• Ion-Dipole – responsible for salts dissolving in water

  
  

  r

  δ-

  δ+

  hydrogen bonds

• Hydrogen Bond (dipole-dipole) – between two pola molecules

  R C O-   δ+

  OH

  O

  Na+      δ-

  H

  H

  Intermolecular Bonds

  
  

  -

  +

  -

  +

• Van der Waals Forces – induced or temporary dipole occurring as non- polar compounds

approach each other F

– a type of dipole- dipole interaction.

Dielectric constant

• Dielectric constant (ε): A measure of a substance’s ability to insulate opposite charges from each other.

• As a measure of solvent polarity, higher ε = higher polarity, greater ability to stabilize charges.

• (In physics, dielectric constant is also known as permittivity or relative permittivity referring to the ability of a substance to attenuate the transmission of an electrostatic force from one charged body to another, compared to a vacuum for which ε = 1)

ε > 15 = polar solvent

ε < 15 =nonpolar solvent

Polar vs. Nonpolar Solvents

Non- polar

Polar aprotic

Polar protic

Longer hydrocarbon chain  less polarity

**

**

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Miscibility of Solvents with Water

http://www.sigmaaldrich.com/Area_of_Interest/Research_Essentials/Solvents/Solvents_Site_Map.html

**

** **

Miscibility of Solvents with Hexane

http://www.sigmaaldrich.com/Area_of_Interest/Research_Essentials/Solvents/Solvents_Site_Map.html

Solubility of Alcohols[ CnH2n+1OH ]

Solubility of Carboxylic Acids

A

B

Solvents are H2O and CCl4 Density of CCl4 is 1.59 g/cm3

Solutes are I2 and KMnO4

Which solvent is the upper layer?

Which flask has I2 added ?

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch15/solut.php

How will the (coloured) compounds partition?

Oil/vinegar mixture

Herb infused oils

Partition Coefficient

• The partition (or distribution) coefficient is the ratio of the concentrations of a compound in two immiscible solvents at equilibrium

  – ie. it is a measure of the difference in solubility of the compound between these two solvents.

  
  

• Often the hydrophobic solvent octanol and water are used.

  [solute]octanol

  [solute]un-ionized

  water

• logPow is a measure of hydrophobicity (or lipophilicity):

log Poctanol/water

= log

log Pow Values

(note: log10(10)=1; log10(1)=0; log10(0.1)= -1; etc)

CH3

log Pow Values of mostly lipophilic food components

1.87 2.26 3.5 5.1

INCREASING experimental log P values >>>>>>>>

log Pow Values of mostly hydrophilic food components

2.13 -1.85 -1.76 -3.24

DECREASING experimental log P values >>>>>>

Soluble in what ??

Solubility of CO2 in water

Figure 5–Fraction of dissolved carbon dioxide, carbonic acid, bicarbonate, and carbonate in water, as a function of pH (adapted from Daniels and others 1985).

Solubility of CO2 in a can

Henry’s Law:

The solubility of a gas in a liquid is proportional to the pressure of gas above that liquid

Picture from Chemistry Structures of Life, K.Timberlake, 2010

Simple Method for Measurement of CO2 in a food

Figure 6–Example of experimental set-up to measure carbon dioxide content in liquid or solid food, and associated reactions (adapted from Gill 1988).

Measurement of O2 also possible (eg using luminescence sensors) but interactions with the food must be considered)